Comparing with the heavy and bulky stored hydrogen sources, such as, compressed hydrogen cylinder, cryogenic liquid hydrogen tank and metal hydride hydrogen storage canister, that is, hydrogen-on-demand generator that produces hydrogen onsite shows advantages as reliable hydrogen source in high hydrogen content, high portability and flexibility.
Among the different technologies used in onsite hydrogen generations, such as, reformer and electrolyser, hydrolysis of metals or chemical hydrides is more attractive, since usually there is no heavy demand on electricity input or heat input during the hydrogen generation process.
For example, hydrolysis of sodium borohydride is widely studied due to its large theoretical hydrogen content (10.8 wt %), compared with the low hydrogen content of 1.6 to 5 wt % in various stored hydrogen sources.
Sodium borohydride is a thermally stable, hygroscopic, white crystalline solid that releases hydrogen through the following hydrolysis reaction with the assistance of catalysts:NaBH4+2H2O→NaBO2+4H2 
Various hydrogen generating systems have been developed for the production of hydrogen gas from aqueous sodium borohydride solution based on this principle. Such systems typically comprise a fuel tank for storing the sodium borohydride solution, a storage tank for storing the by-product, sodium metaborate (NaBO2) solution produced by the process, a pump, a reactor and a separator. However, all of these have their significant drawbacks.
Firstly, the limited durability of the heterogeneous catalysts leads to higher cost of a hydrogen generator. Secondly, the hydrolysis by-product, sodium metaborate has a relatively small solubility in water, only 28 g in 100 g water at 25° C. It has been reported that the optimum concentration of sodium borohydride in the starting solution is about 15 wt % (the solubility of sodium borohydride in water is approximately 35 wt % at 25° C.), otherwise sodium metaborate would precipitate from the solution and thus restricting the catalyst from reacting with the reactants and clogging the reactor and tubes. As a result, hydrogen generation capacity is usually capped at 3.2 wt %, a value that is rather inadequate considering the theoretical value of 10.8 wt %.
More efforts are made by researchers on searching for methods to use high concentration sodium borohydride as a starting solution; less effort is made on using solid sodium borohydride directly for the hydrolysis reaction. Although using solid sodium borohydride directly seems more superior in hydrogen generation capacity, the main obstacle is the crystallization of the by-product, sodium metaborate, which is sticky and turns strong, thus clogging the surface of the catalyst and unreacted fuel. This in turns retard the contact of the reactants, blocking the tubes and the reactors. This interrupts continuous generation of hydrogen.
Therefore, it is desirable to design a solid sodium borohydride fuel and related hydrogen generator which can constrain the by-product sodium metaborate within certain region and thus ensure a smooth and continuous operation, as well as a uniform reaction in the whole space of the reactor. The hydrogen generation capacity should be significantly enhanced, compared with the system using aqueous sodium borohydride fuel.
Consequently, there is a need to provide an alternative fuel, system and method for generating hydrogen that seeks to address at least some of the problems described hereinabove, or at least to provide an alternative.